High pressure tubular reactors are widely used for the polymerization of ethylene at high pressure, for example, pressures of over 1000 bar, and up to 3000 bar or higher. Such tubular reactors are typically very long, for example more than 1000 meters long and are provided along their length with cooling jackets, injection ports for initiator, ethylene side streams and comonomer, as well as various ancillary equipment such as temperature sensors. Fresh ethylene from an ethylene supply is compressed to reactor pressure typically by a combination of a primary compressor which compresses the ethylene to an intermediate pressure, say, around 300 bar, and a secondary compressor which compresses the fresh ethylene together with recycled ethylene from that intermediate pressure up to the final reactor pressure. The ethylene is then usually heated in a pre-heater to a temperature suitable for the initiators to be used before being fed into the front end of the tubular reactor. As the ethylene flows along the tubular reactor initiator (and optionally comonomer) is injected, usually at several points along the length of the tubular reactor, and the ethylene polymerized to give a mixture comprising principally polymer and unreacted monomer. That mixture leaves the reactor through a valve, which is generally referred to as a high pressure let down valve, and then enters a separation system in which unreacted monomer is separated from the polymer and recycled back to the suction of the secondary compressor.
Various forms of separation system are known. One such known separation system includes two separation vessels arranged in series. The first separation vessel, sometimes referred to as the high pressure vessel, has an inlet for the product mixture coming from the high pressure let down valve, an outlet for the separated unreacted monomer gas (referred to as “off gas”) and an outlet in the bottom of the vessel for the polymer. That polymer, which still contains, say, 30 to 40 wt % of ethylene, passes from the outlet of the first vessel through a conduit into the second vessel, often referred to as the low pressure vessel, where almost all of the remaining ethylene separates off. That off gas is removed through an upper outlet as off gas leaving the molten polymer to flow through an outlet in the bottom of the vessel. Typically, the high pressure vessel will operate at a pressure such that the off gas can be returned, via a recycle system, to the suction of the secondary compressor. The low pressure separator operates at a much lower pressure, and the off gas from the low pressure separator must be compressed in a further compressor (known as a “purge compressor” which may be part of the primary compressor) before being sent to the secondary compressor.
The molten polymer leaving the separation system is generally extruded and cooled to give a solid product, typically in pellet form, which is sent for storage or to other product handling facilities.
The polymerization of ethylene is an exothermic process which generates heat thereby causing the temperature of the monomer/polymer mixture to rise to a peak downstream of an initiator injection point and then to fall under the influence of the cooling applied to the tubular reactor. Where there are several initiator injection points along the length of the reactor the temperature of the monomer/polymer mixture will rise and fall several times as it travels along the length of the reactor, such that the temperature profile of the mixture along the length of the reactor is an undulating curve. Cooling of the product mixture in the tubular reactor is hindered by the necessarily thick walls of the reactor and effective cooling therefore requires long sections of cooling jacket. Another factor hindering cooling is the formation of a lamina flow region adjacent the inside wall of the tubular reactor which inhibits heat transfer out of the reactor. That laminar flow region increases in thickness as the viscosity of the product mixture increases along the length of the reactor, and is particularly significant when a high viscosity grade of polymer is being produced. In order to disrupt that region of laminar flow, some tubular reactors are bumped, that is, the high pressure let down valve is briefly opened up at regular intervals to give a sudden increase in the flow rate through the reactor and thereby improve heat transfer out of the reactor.
In some polymerization plants the product mixture is further cooled after it has left the tubular reactor by injecting cold ethylene into it as a “quench”, prior to the entry into the separation system. The use of a “quench”, however, has some disadvantages. In particular, the cold ethylene must be compressed in order to raise it to a pressure at which it can be injected into the flow of product mixture from the reactor. That compression requires energy and the compressor has also an associated maintenance cost.
U.S. Pat. No. 4,255,387 focuses on the use of rupture discs in an HPPE reactor with possible use of a bump cycle. However the use of a jet pump to introduce a fluid flow into the product mixture is not mentioned. GB2134121 addresses use of an anti-foam agent in the production of catalytically polymerized linear low density polyethylene, as opposed to a non-linear low density polyethylene material produced by the use of free radical initiators. The invention specifically addresses polyethylene production using an initiators injection system. U.S. Pat. No. 3,714,123 describes high pressure polymerization using bumping; however the use of a jet pump is not mentioned. U.S. Pat. No. 4,027,085 describes control of the peak temperature in the reactor the use of pulsing. However, the use of jet pumps to optimize operation is not disclosed.